A wheel bearing apparatus, which is also referred to as a hub bearing, is for supporting a wheel of an automobile. There are a bearing apparatus for a driving wheel and a bearing apparatus for a driven wheel.
A wheel bearing apparatus has an inner member and an outer member which are relatively rotatable via rolling elements. One of the inner member and the outer member is fixed to a vehicle body, and the other is attached to a wheel. Accordingly, the member fixed to the vehicle body is the fixed side, and the member attached to the wheel is the rotating side. A wheel bearing apparatus having its inner member attached to the wheel and having its outer member attached to the vehicle body is referred to as an inner ring rotation type. A wheel bearing apparatus having its inner member fixed to the vehicle body and having its outer member attached to the wheel is referred to as an outer ring rotation type. In both of the types, the member attached to the wheel has a hub flange. Using hub bolts implanted into the hub flange and hub nuts, the wheel are fixed to the flange.
The wheel bearing apparatus for a driving wheel is the inner ring rotation type because it must transmit power to the driving wheel. That is, the inner member having the hub flange for attaching the wheel is rotatably supported by the outer member fixed to a knuckle, and the inner member is coupled to a drive shaft.
The wheel bearing apparatus for a driven wheel may be any of the inner ring rotation type and the outer ring rotation type. When it is the inner ring rotation type, the inner member is provided with the hub flange, and the inner member is rotatably supported by the outer member fixed to the vehicle body. When it is the outer ring rotation type, the outer member is provided with the hub flange, and the outer member is rotatably supported by the inner member fixed to the vehicle body.
A double row angular contact ball bearing with low torque characteristics is popularly employed as the wheel bearing apparatus for its desirable bearing rigidity, durability withstanding misalignment, and improved fuel efficiency. In a double row angular contact ball bearing, balls in a plurality of rows are interposed between a bearing inner ring (inner race) and a bearing outer ring (outer race). The rings are in contact with the rows of balls at a predetermined contact angle. The inner member corresponding to the bearing inner ring has two rows of raceway grooves along its outer circumference, and the outer member corresponding to the bearing outer ring has two rows of raceway grooves along its inner circumference. Between the opposing paired raceway grooves, a plurality of balls roll. The rows of rolling elements are retained at a predetermined interval in the circumferential direction by retainers.
In some cases, while the depth of a raceway groove, that is, the groove depth, is also referred to as a shoulder height, they are not exactly synonymous with each other. That is, the shoulder height is the distance from the bottom of a raceway groove to the upper surface of the shoulder (which is the outer diameter surface in relation to the inner member, and the inner diameter surface in relation to the outer member). On the other hand, the groove depth is the value obtained by subtracting, from the shoulder height, any chamfer, bevel, or auxiliary raceway surface provided at the surface of the raceway groove, that is, at the edge of the raceway surface. Accordingly, normally, the groove depth assumes a value smaller than the shoulder height. Here, the auxiliary raceway surface is a pseudo-raceway surface provided at the edge of the raceway surface so as to address an excessive load capacity. The cross-sectional shape of the raceway groove is an arc-shape. While the auxiliary raceway surface is continuous from the arc of the raceway groove, the auxiliary raceway surface is not formed by an extension of the arc of the identical curvature.
Patent Literature 1 discloses, in paragraph 0006, an auxiliary raceway surface that smoothly continues from an arc-shaped curved line forming the cross section of a raceway groove. Provision of such an auxiliary raceway surface is expected to exhibit the following effects. That is, when a great moment load is put on the bearing and the contact angle increases, the contact ellipse is pushed out from the raceway groove to the auxiliary raceway surface. However, since the auxiliary raceway surface smoothly continues from the arc-shaped curved line that forms the raceway groove, generation of the edge load is not invited despite the contact ellipse being pushed out to the auxiliary raceway surface. Further, the inclination of the auxiliary raceway surface is greater than that of a surface being just an extension of the raceway surface. That is, since an inclination angle of certain degrees can be secured, the auxiliary raceway surface will not be ground with the side surface of a grinding wheel during grinding work. This avoids an increase in the grinding work hours.
Patent Literature 2 discloses, in paragraphs 0005 to 0008, an auxiliary raceway surface that is provided at the edge of a raceway groove and smoothly continues from an arc-shaped curved line forming the raceway groove. The cross section of the auxiliary raceway surface is formed by a curved line with a curvature smaller than that of the arc-shaped curved line or by a straight line. While the effects exhibited by the auxiliary raceway surface are substantially similar to those of Patent Literature 1 as described above, in Patent Literature 2, a chamfered portion having an arc-shaped cross section being continuous to the edge of the auxiliary raceway surface is further formed. Thus, the edge load of the contact ellipse is further alleviated.
Conventionally, from the aspects of functions and works, a ratio h/d of a groove depth h against a ball diameter d is h/d<0.50 in relation to the inner ring and h/d<0.40 in relation to the outer ring. With an h/d exceeding 0.50, in the cross section of the raceway groove, the sidewall of the raceway groove is inwardly warped, whereby grinding becomes difficult. Further, in relation to the outer ring, the upper limit of h/d is 0.40.
The reason why the upper limit of the groove depth of the raceway groove is 0.40 d with the outer ring while the upper limit is 0.50 d with the inner ring is explained as follows. As to the inner ring, grinding work can be performed with an h/d exceeding 0.50, depending on the manner of abutting a grinding wheel. However, considering the number of work steps, it is practically difficult to achieve h/d>0.50. As to the outer ring, double rows of raceway grooves are simultaneously ground for higher precision. The only solution for achieving the simultaneous high precision grinding is plunge grinding using a form grinding wheel whose contour matches with the cross-sectional shape of the raceway grooves and whose feed is the cutting direction. Accordingly, it is difficult to form deep grooves, and an h/d of 0.40 is the limit.
Note that, H/d>0.50 in relation to the inner ring can be achieved by provision of the above-described auxiliary raceway surface. Here, reference character H represents the shoulder height including the auxiliary raceway surface, and reference character h represents the groove depth not including the auxiliary raceway surface. That is, conventionally, while there do not exist bearings attaining h/d>0.50, there exist bearings attaining H/d>0.50 (see Patent Literature 2).